For many years a variety of manual (non-electric) sharpeners have been developed and sold using interdigitating, comb-like abrasive structures as described in U.S. Pat. Nos. 5,390,431 and 5,582,535.
Although this technology has been effective and broadly accepted by consumers, it had a number of limitations. First, the motion of the blade against the abrasive surfaces created a pattern of grooves along the edge facet that were parallel to the apex of the edge. These grooves tend to undercut and weaken the metal supporting the edge. Since the predominant mechanism of edge failure is the edge curling and folding over on itself, these grooves tend to reduce the durability of the edge.
Secondly, the finishing stage of these type of sharpeners, when using very fine grits of abrasives, develop an edge apex that is uninterrupted and perfectly linear. Although this type of edge is effective for many cutting functions, it was discovered that edges with micro serration are more effective for many household and outdoors applications. However, achieving a microserrated edge with interdigitating comb-like structures would be very difficult and costly.